1. Field of the Invention
This invention collects and analyzes condensable gases using a cryogenic cooling device connected with a charcoal cold trap assembly. More particularly, it uses a removable cryogenic cooling device instead of liquid nitrogen to trap condensable gases.
2. Background and Description of Related Art
When irradiated fuel samples are annealed at high temperatures, it is necessary to obtain a quantitative evaluation of the radioactive fission gas that is released. A cold trap can be used to condense, collect, and analyze this gas, in which the hot gas is transported from the high temperature area by a carrier or purge gas to a cold trap assembly filled with a high-surface-area substance, preferably activated charcoal. In the prior art, the cold trap is cooled with liquid nitrogen. The cooled charcoal condenses and xe2x80x9ctrapsxe2x80x9d the hot, radioactive gas. A radiation detector, in close proximity to the trap monitors when and how much radioactive condensable gas has been trapped. A thermocouple may be used to monitor the temperature of the hot gases. A radiation source may be used to calibrate the radiation detector. A vacuum canister insulates the cold trap. After analyses are complete, the charcoal then must be heated to drive off condensed products from the charcoal. The trap may be heated by flowing warm gas, such as air, through the air inlet/outlet tube. In a particularly preferred embodiment, a thin, flexible, electric heating element is inserted in the air inlet/outlet tube to heat the cold trap canister.
Cooling with liquid nitrogen can be problematic. The lines are susceptible to freezing, thus limiting the flow of the liquid. This can cause the cold trap to heat up and evaporate or not condense and trap the gas, leading to lost data. The venting of nitrogen in confined, inhabited spaces is also a concern. Additionally, the liquid nitrogen cooled traps usually use a great amount of liquid nitrogen, making maintaining an adequate supply a problem.
Most cold traps are a single welded assembly. Therefore, the entire cold trap assembly must be treated as radioactive material for disposal, which can be very costly. The design of the present invention reduces the components exposed to radioactivity, thus minimizing the amount of material that must be treated as radioactive waste.
It is an object of the present invention to cool the cold trap with a removable cryogenic cooling device that does not become radioactively contaminated under normal operating circumstances.
It is another object of the present invention to use a cryogenic cooling device, rather than liquid nitrogen, to cool the cold trap.
It is yet another object of the present invention to limit the exposure to radiation to the cold trap, so that the vacuum canister, and removable cryogenic cooling device may be reused.
The present invention uses a cold trap that is cooled by an off-the-shelf, self-contained cryogenic cooling device, instead of liquid nitrogen. The trap has four major components: (1) a removable cryogenic cooling device (2) a vacuum canister, (3) a cold trap canister filled with activated charcoal, and (4) a cover assembly with welded inlet/outlet tubes and an o-ring seal between the cover assembly and the vacuum canister.
The removable cryogenic cooling device is a closed cooling system capable of keeping the cold trap canister cool enough to condense the hot gas, near liquid nitrogen temperatures, around xe2x88x92150xc2x0 C. In a particularly preferred embodiment, an integral portable compressor is connected to the cryogenic cooling device with a flexible metal hose. The cryogenic cooling device is removable and is secured to the bellows with standard vacuum fittings. Because the cold trap canister is a sealed assembly, the cryogenic cooling device does not become radioactively contaminated under normal operating conditions, and can be removed and reused for other analyses.
The cover assembly has a flexible vacuum bellows welded to it, to which the cryogenic cooling device is attached using standard vacuum connections. Stainless steel tubes are attached to the cover assembly for purge gas inlet and outlets, a heated air inlet/outlet, a thermocouple, and a vacuum port.
The cold trap canister portion of the invention comprises a cylindrical canister with gas diverters radiating from its inner surface. The canister can be made from any conductive metal that does not react with the process gases and does not affect the seal integrity of the welded tubes. In a particularly preferred embodiment, the canister is made of copper and is filled with activated charcoal. The canister also has inlet and outlet tubes for purge gas flow.
The vacuum canister can be made from any durable material that can withstand a vacuum. In a particularly preferred embodiment, the vacuum canister is a modified stainless steel beaker. A cover assembly is bolted to the vacuum canister with an o-ring seal between the cover assembly and vacuum canister.